The present invention relates to dilatation balloon catheters employed in applications such as percutaneous transluminal angioplasty (PTA) and percutaneous transluminal coronary angioplasty (PTCA) procedures, and more particularly to enhancements to such catheters and their dilatation balloons for improved maneuverability in smaller and more tortuous passages of the vascular system.
Dilatation balloon catheters are well known for their utility in treating the build-up of plaque and other occlusions in blood vessels. Typically a catheter is used to carry a dilatation balloon to a treatment site, where fluid under pressure is supplied to the balloon, to expand the balloon against an obstruction. Additionally, the expansion of the balloon may deploy a stent device in the treatment area.
The dilatation balloon is typically mounted along the distal portion, e.g., distal end region, of the catheter and coaxially surrounds the catheter shaft. Upon expansion of the dilatation balloon, the main body portion or medial section, of the balloon, sometimes referred to as the working portion, increases to define a diameter which is substantially larger than the diameter of the catheter shaft. Proximal and distal sleeves or stems of the balloon have diameters substantially equal to the diameter of the catheter. Proximal and distal tapered sections, or cones, join the medial region to the proximal and distal shafts, respectively. Each cone diverges in the direction toward the medial region. Bonds between the balloon and catheter form a fluid tight seal to facilitate dilatation of the balloon by introduction of a fluid under pressure.
Along with body tissue compatibility, primary attributes considered in the design and fabrication of dilatation balloons are strength and pliability. A higher hoop strength or burst pressure generally reduces the risk of accidental rupture of a balloon during dilatation, although this is also dependent on the characteristics of the vessel lesion.
Pliability refers to formability into different shapes, rather than elasticity. In particular, when the balloon is an uninflated configuration, the dilatation balloon is evacuated, flattened and generally wrapped circumferentially about the catheter distal region. Thin, pliable dilatation balloon walls facilitate a tighter wrap that minimizes the combined diameter of the catheter and balloon during delivery. Furthermore, pliable balloon walls enhance the catheter “trackability” in the distal region, i.e. the capability to bend in conforming to the curvature in vascular passages.
One method of forming a strong and pliable dilatation balloon of polyethyleneterephthalate (PET) is disclosed in U.S. Pat. No. Re. 33,561 (Levy). A tubing of PET is heated at least to its second order transition temperature, then drawn to at least triple its original length to axially orient the tubing. The axially orientated tubing is then radially expanded within a cylindrical form, to a diameter at least triple the original diameter of the tubing. The form defines the aforementioned main body, shafts and cones, and the resulting balloon has a burst pressure of greater than 200 psi.
Such balloons generally have a gradient in wall thickness along the cones. In particular, dilatation balloons with an expansion diameter in the range of 3.0-4.0 mm tend to have a wall thickness along the main body in the range of 0.0004-0.0008 inches (0.010-0.020 mm). Near the main body, the cones have approximately the same wall thickness. However, the wall thickness diverges in the direction away from the main body, until the wall thickness near each shaft is in the range of 0.001-0.0025 inches (0.025-0.063 mm). Smaller dilatation balloons (1.5-2.5 mm) exhibit the same divergence in the cone walls, i.e. from 0.0004-0.0008 inches near the main body to 0.0008-0.0015 inches (0.02-0.04 mm) near the associated shaft or stem.
The increased wall thickness near the stems does not contribute to balloon hoop strength, which is determined by the wall thickness along the balloon medial region. Thicker walls near the stems reduce maneuverability of the balloon and catheter. The dilatation balloon cannot be as tightly wrapped, meaning its delivery profile is larger, limiting the capacity of the catheter and balloon for treating occlusions in smaller vessels.
U.S. Pat. No. 4,963,133 (Noddin) discloses an alternative approach to forming a PET dilatation balloon, in which a length of PET tubing is heated locally at opposite ends and subjected to axial drawing, to form two “necked down” portions which eventually become the opposite ends of the completed balloon. The necked down tubing is simultaneously axially drawn and radially expanded with a gas. The degree to which the tubing ends are necked down is said to provide control over the ultimate wall thickness along the tapered walls (or cones), so that the wall thickness can be equal to or less than the wall thickness along the main body. This approach, however, is said to result in a comparatively low burst pressure, only about 8 atmospheres, or about 118 psi.
Typically, the balloon is secured to an elongate member of the catheter shaft. In this manner, the proximal and distal sleeves of the balloon is secured for example, heat welded to a catheter shaft. The bonded area of the balloon often gets stiffer and larger in diameter after bonding to the catheter shaft. This is partly due to material accumulation, and to crystallization of the polymer material of the balloon.
Therefore, it is an object of the present invention to provide a dilatation balloon having a high burst pressure, high hoop strength, good folding characteristics, and good trackability.
A further object is to provide a balloon with portions of the balloon wall selectively thinned to enable a tighter wrapping of the balloon circumferentially about a catheter distal end region, for a reduced profile during balloon delivery.
Yet another object is to provide a process for selectively removing material from a balloon catheter and its dilatation balloon, to enhance catheter trackability and maneuverability without crystallization, embrittlement or other thermal degradation of material. This selective removal of material may be achieved through the use of an excimer laser. This selective removal of material may also be achieved through the use of a phemto-second laser, or any other laser source that is capable of material removal. This selective removal of material may also be achieved through the use of a mechanical process such as drilling, milling, blasting, etching, or grinding.